In the prior art, an amide compound is synthesized from a compound having an acyl fluoride group within a molecule through several routes, for example, by direct reaction of the acyl fluoride group-bearing compound with an amino compound. It is known that this direct reaction proceeds in a quantitative manner. During the reaction, hydrogen fluoride is produced as a by-product. If the amino compound is present in excess in the reaction system, hydrogen fluoride forms a salt with the amino compound.
Separately, another amino compound is added to the reaction system as an acid acceptor if necessary. Hydrogen fluoride can also form a salt with the other amino compound which is optionally added. Then, to remove the salt of hydrogen fluoride with amino compound (hydrofluoric acid amine salt, hereinafter) from within the reaction system and isolate the target amide compound, post-treatment by water washing is generally carried out.
However, some amide compounds produced are not effectively separable from water. Then the prior direct reaction process has the problems that time-consuming many steps are necessary until the target amide compound is obtained, and incomplete purification allows a large amount of hydrofluoric acid amine salt to remain in the amide compound product, which eventually invites a decline of production yield. Even with a target amide compound that can be isolated by distillation, if the removal of hydrofluoric acid amine salt in the preceding step is incomplete, the hydrofluoric acid amine salt will sublimate during the distillation step so that the distilled fraction is contaminated with it. Sometimes, the hydrofluoric acid amine salt will precipitate on a cooling section of a distillation column, clogging the distillation column.
As a means of avoiding these problems, Japanese Patent No. 2,855,081 discloses to add a silazane compound to a reaction mixture resulting from reaction of a compound having an acyl fluoride group within a molecule with an amino compound and containing the amide compound and hydrogen fluoride and/or a salt of hydrogen fluoride with the amino compound, for thereby removing the hydrogen fluoride and/or the salt of hydrogen fluoride with the amino compound.
Although this method is superior to the previous water washing method, it is difficult to remove unreacted silazane compound and partially reacted silazane compounds remaining in the system at the end of reaction, which mostly cause the target amide compound to be colored. The silazane compound used in this method is generally expensive and sensitive to air-borne moisture and thus difficult to handle.
It would be desirable to have an amide compound purifying method capable of recovering an amide compound, from a reaction mixture resulting from reaction of an acyl fluoride group-bearing compound with an amino compound, through simple steps and in a high yield and high purity.